Development of ultra-linear traveling wave tubes for telecommunications applications

Traveling wave tubes (TWT) intended for telecommunications applications are increasingly required to have high linearity, good intermodulation performance, and high power levels; especially for use as modern multi-carrier power amplifiers (MCPA) and in systems that use digital signal encryption. The linearity of the TWT is greatly improved by optimization of the taper of the helix circuit to reduce the gain compression and phase shift of the output signal as the drive level is increased, and by operation of the tube significantly backed off from saturation in the linear region of the AM/AM and AM/PM transfer curves. However, backed off operation at a given output power level requires increased beam current and power handling capabilities compared to a design operating at saturation, and also results in a greatly reduced efficiency of the TWT. At Hughes, we have developed L-band and S-band TWTs for telecommunication applications that provide good intermodulation performance and high efficiency compared to solid-state amplifiers. The L-band model TWT normally operates 8 dB backed off from 1600 W at saturation, and produces over 250 W of continuous output power with an 8-tone continuous-random-phase signal intermodulation level of -28 dBc and an efficiency of over 25%. The S-band model TWT normally operates 6 dB backed off from 2000 W at saturation, and has produced over 500 W of continuous output power at an efficiency of over 35%.